feat(planners): add standardized planner content blocks

[vaibhav-patel]

What

Adds google.adk_community.planners — a read-only, additive helper that converts planner-produced google.genai.types.Part lists into LangChain v1-style typed content blocks.

• parts_to_content_blocks(parts) / part_to_content_block(part) return typed blocks with a type discriminator:
  • reasoning — from thought=True parts; carries reasoning text and a reasoning_kind (planning / replanning / reasoning / action, derived from PlanReActPlanner inline tags, or None for BuiltInPlanner thinking).
  • text — from non-thought text parts (e.g. a final answer).
  • tool_call — from parts carrying a function_call (name, args, id); takes precedence over incidental text.
• Read-only: never mutates input parts; skips parts with no mappable content; preserves order.
• Self-contained: depends only on google-genai (transitively present via google-adk). No pyproject.toml dependency changes; flit already packages src/**. Registered in the top-level __init__.py alongside the other flat modules.
• Ships with its own README.md and 21 unit tests.

Closes #162.

Why

Planner reasoning is currently encoded as thought=True plus inline /*PLANNING*/-style tags that every consumer has to re-parse. This gives them a stable, provider-agnostic typed view instead. Redirected from core ADK PR google/adk-python#6185 (closed; maintainer asked for it as a community module).

Testing Plan

• Unit tests: added tests/unittests/planners/test_planner_content_blocks.py (21 tests) covering each block type, the PlanReActPlanner tag→reasoning_kind mapping, leading/trailing tag stripping, empty/redacted/unmappable parts being skipped, order preservation, the read-only guarantee (input parts unchanged, returned args is a copy), and round-trips through the real PlanReActPlanner.process_planning_response / BuiltInPlanner output.
• Formatting/typing: isort (profile=google) and pyink (line-length 80) report clean on the new files; mypy (repo config) reports no issues on the src files.

$ pytest tests/unittests/planners/ -q
.....................                                                    [100%]
21 passed, 5 warnings in 0.31s

(The 5 warnings are pre-existing google-adk deprecation notices, unrelated to this change.)